The present invention relates to an apparatus offering extruded honeycomb structures with improved dimensional accuracy, and more particularly to an apparatus for overcoming the problem of bow in an extrudate.
Ceramic honeycomb substrates for automotive applications are generally produced by cutting and firing individual pieces from a stream of honeycomb extrudate, or by cutting the pieces from a dried green or fired ceramic xe2x80x9clogxe2x80x9d of extrudate which may be of meter or greater length. To meet customer requirements for the subsequent catalyst coating and xe2x80x9ccanningxe2x80x9d of these substrates in suitable metal enclosures, it is important that the logs and pieces cut from the logs have sides which are absolutely straight and parallel.
The production of a straight stream of extruded material is quite difficult; in most cases at least some xe2x80x9cbowingxe2x80x9d of the extrudate, attributable to uneven flow of material through the extrusion die, is observed. This bowing can be caused by non-uniform flow characteristics in the batch, but more commonly is due to uneven flow resistance across the face of the extrusion die. Even with careful attention to die fabrication, uneven machining resulting from facts such as progressive tool wear, misalignment of feed holes and discharge slots, and non-uniform exposure to chemical machining and/or plating electrolytes often result in at least some bowing tendency being xe2x80x9cbuilt inxe2x80x9d to most honeycomb extrusion dies during manufacture.
One prior art approach to the resolution of this problem, as disclosed in U.S. Pat. No. 6,039,908 involves the use of a bow deflector which comprises two aperture plates which co-act to alter the flow of the extrudable material to the die. Each of the aperture plates are tapered, varying in thickness from one edge to the other, and each comprises apertures through its thickness aligned in a direction parallel to the extrusion axis. Each aperture plate is independently rotatable about the extrusion axis with respect to both the die and the other aperture plate. Rotating the tapered plates relative to each changes the amount of bow correction available, by increasing or decreasing the differential in total aperture length between the longest and shortest aperture sets across the bow deflector.
The end effect of inserting this device in front of the extrusion die is that the pressure and feed rate of extrudable material to each portion of the die will be inversely proportional to aperture length in the bow deflector behind that portion. This produces a flow velocity gradient across the diameter of the bow deflector in the direction of maximum taper. Given proper alignment of the bow deflector with respect to the die, the flow gradient from the deflector can theoretically counterbalance a pre-existing flow gradient from the die, resulting in an extruded log with much less bend or bow.
While this approach is sound in theory, problems have been identified in practice and include high maintenance costs from clogged and wore-down apertures causing extrudate flow impedance, and swollen webs in the extruded honeycomb structures resulting in defects and product failure. Due to the intricacy of the design of the prior art device cost is also an issue.
The present invention overcomes the problem of bow in a honeycomb extrudate through the use of a bow correction device of improved design. That design permits bow correction to be made over a relatively wide range of bowing conditions, and without any need to change bow deflectors.
In a first aspect, the invention includes improved apparatus for the extrusion of honeycomb structures which comprises, in combination, a honeycomb die of any suitable design (which need not be bow-corrected) and a bow correction device having a singular aperture which acts to alter the flow of extrudable material to the die in an exact and controllable manner.
As is conventional, the honeycomb extrusion die employed in the apparatus of the invention has an inlet face comprising a plurality of feed holes and an outlet face comprising a discharge opening. The discharge opening is configured in the conventional manner to produce an extrudate of honeycomb configuration from a flow stream of plastic material flowing downstream through the die along an extrusion axis parallel with the direction of extrusion.
The bow deflector, which is positioned in the flow stream immediately upstream of and adjacent the inlet face of the die, comprises a base having an aperture through its thickness aligned in a direction parallel to the extrusion axis, and a slide plate movably mounted to the base. The slide plate moves relative to the base plate controlling the diameter of the aperture.
Controlling the diameter of the aperture will change the amount of bow correction available, while changing the position of the slide plate relative to die the extrusion apparatus, will change the direction of the bow, such that the flow velocity gradient across the face of the feed stream exiting the deflector device and entering the extrusion die may be controlled as to both magnitude and direction.
Another aspect of the invention comprises an improved method for extruding a honeycomb structure from a plasticized powder batch material. In accordance with that method, a feed stream of the plasticized batch material is first directed along an extrusion path through an aperture in a device disposed across the feed stream. The deflector device has a flow resistance which varies across its length, thereby impressing a flow velocity or pressure drop gradient across the face of the feed stream traversing the deflector device. The result of this superimposed pressure drop is that a flow velocity gradient is developed across the face of the feed stream by passage through the deflector device. The feed stream with the flow velocity gradient is directed into and through the honeycomb extrusion die.